The desire to accurately determine a position of a subject's eye manifests itself in many fields of endeavor. For example, in opthalmic applications, knowledge of a position of an eye is desirable to make eye measurements or to perform refractive surgery.
Various apparatus for determining a position of a subject's eye are known. FIG. 1 is a schematic illustration of a first conventional apparatus for determining a position of a subject's eye. A first laser 10 and a second laser 20 are aligned so that the beams from the lasers intersect at a location L. The subject's eye is brought to a longitudinal location L (in the y-direction) by observing an image of the light in the beams that is scattered by the subject's cornea C using a camera 50. When two spots R1 and R2 are observed by the camera, the subject's eye is either in front of or behind location L; and when a single spot is observed the subject's eye is at the location. Such an arrangement is not sensitive to lateral displacement of the eye (i.e., in the x-direction or the z-direction).
FIG. 2 is a schematic illustration of another apparatus for determining a position of a subject's eye. Using the apparatus of FIG. 2, a first point source 60 and second point source 70 are arranged to project light onto a subject's eye. A camera is positioned to image light reflected from the subject's cornea.
Because the cornea operates as a convex mirror surface, the camera 75 observes virtual images S1′ and S3′ of the point sources. The eye can be brought to a known location by moving the eye so that images S1 and S3 are positioned symmetrically about a centerline corresponding to optical axis OA. The apparatus in FIG. 2 determines a lateral position (i.e., in the x-direction and the z-direction) of the eye. However, the technique is not sensitive to displacement along the longitudinal axis (i.e., in the y-direction).
While the above apparatus provide some knowledge of eye position, what is needed is a device that can provide more accurate position information of an eye and/or a more facile technique of determining position.